background image

AIM

10/12/17

7

−1−58

Meteorology

relative humidity is low in any layer between the
surface and 15,000 feet. Then the lower altitudes may
be characterized by strong out flowing winds and
severe turbulence.

e.

The probability of lightning strikes occurring to

aircraft is greatest when operating at altitudes where
temperatures are between minus 5 degrees Celsius
and plus 5 degrees Celsius. Lightning can strike
aircraft flying in the clear in the vicinity of a
thunderstorm.

f.

METAR reports do not include a descriptor for

severe thunderstorms. However, by understanding
severe thunderstorm criteria, i.e., 50 knot winds or

3

/

4

inch hail, the information is available in the report

to know that one is occurring.

g.

Current weather radar systems are able to

objectively determine precipitation intensity. These
precipitation intensity areas are described as “light,”
“moderate,” “heavy,” and “extreme.”

REFERENCE

Pilot/Controller Glossary

− Precipitation Radar Weather Descriptions

EXAMPLE

1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between
ten o’clock and two o’clock, one five miles. Precipitation
area is two five miles in diameter.

2. Alert provided by an FSS:
(aircraft identification) EXTREME precipitation two zero
miles west of Atlanta V

−O−R, two five miles wide, moving

east at two zero knots, tops flight level three niner zero.

7

−1−29. Thunderstorm Flying

a.

Thunderstorm Avoidance. Never regard any

thunderstorm lightly, even when radar echoes are of
light intensity. Avoiding thunderstorms is the best
policy. Following are some Do’s and Don’ts of
thunderstorm avoidance:

1.

Don’t land or takeoff in the face of an

approaching thunderstorm. A sudden gust front of
low level turbulence could cause loss of control.

2.

Don’t attempt to fly under a thunderstorm

even if you can see through to the other side.
Turbulence and wind shear under the storm could be
hazardous.

3.

Don’t attempt to fly under the anvil of a

thunderstorm. There is a potential for severe and
extreme clear air turbulence.

4.

Don’t fly without airborne radar into a cloud

mass containing scattered embedded thunderstorms.
Scattered thunderstorms not embedded usually can
be visually circumnavigated.

5.

Don’t trust the visual appearance to be a

reliable indicator of the turbulence inside a
thunderstorm.

6.

Don’t assume that ATC will offer radar

navigation guidance or deviations around thunder-
storms.

7.

Don’t use data-linked weather next genera-

tion weather radar (NEXRAD) mosaic imagery as the
sole means for negotiating a path through a
thunderstorm area (tactical maneuvering).

8.

Do remember that the data-linked NEXRAD

mosaic imagery shows where the weather was, not
where the weather is. The weather conditions may be
15 to 20 minutes older than the age indicated on the
display.

9.

Do listen to chatter on the ATC frequency for

Pilot Weather Reports (PIREP) and other aircraft
requesting to deviate or divert.

10.

Do ask ATC for radar navigation guidance

or to approve deviations around thunderstorms, if
needed.

11.

Do use data-linked weather NEXRAD

mosaic imagery (for example, Flight Information
Service-Broadcast (FIS-B)) for route selection to
avoid thunderstorms entirely (strategic maneuver-
ing).

12.

Do advise ATC, when switched to another

controller, that you are deviating for thunderstorms
before accepting to rejoin the original route.

13.

Do ensure that after an authorized weather

deviation, before accepting to rejoin the original
route, that the route of flight is clear of thunderstorms.

14.

Do avoid by at least 20 miles any

thunderstorm identified as severe or giving an intense
radar echo. This is especially true under the anvil of
a large cumulonimbus.

15.

Do circumnavigate the entire area if the area

has 6/10 thunderstorm coverage.

16.

Do remember that vivid and frequent

lightning indicates the probability of a severe
thunderstorm.